1. Field of the Invention
The present invention relates generally to an MOS transistor, and in particular to an MOS transistor having gate electrodes arranged at corresponding channel zones.
2. Description of the Related Art
MOS field effect transistors which have two gate electrodes are known and are commonly referred to as MOS tetrodes. MOS tetrodes are distinguished by good high-frequency properties and they are often used in, for example, pre-stages of television tuners where the supply voltage typically is approximately 12 volts.
It is desired in the pre-stages of television tuners that the integrated drain current be stabilized. Integrated circuits (IC) are typically provided for this purpose, wherein the main constituent of the circuit is an MOS tetrode. It is desired that the integrated circuits operate at the voltages which are available in traditional television tuners. It is required as a secondary condition, for example, that the second gate control voltages which range from approximately one volt to eight volts for the tetrode be available in television tuners. A prerequisite for the use of such a component is the availability of the relatively high second gate cutoff voltage. To achieve a high second gate cutoff voltage, a doping concentration at the, for example, p-conductive second gate channel zone must be higher than that of the base material which is, for example, p-doped silicon. This high doping concentration is achieved by appropriate boron implantation, for example.
Tunnel punch-through occurs at relatively low drain voltages at the pn-junction of the tetrode when relatively high p and n concentrations lie under a gate electrode insulated only by a thin gate oxide layer, such as under a second gate electrode neighboring the drain electrode. For example, an MOS tetrode can only work given drain voltages up to nine volts with a second gate cutoff voltage higher than one volt, in the known MOS tetrode. Such a component should be usable at drain voltages of up to 13 volts, however, as is present in, for example, television tuners.